Drops terms in a `REML`

fixed model from a Genstat regression (R.W. Payne).

### Options

`PRINT` = string tokens |
Controls printed output (`model` , `deviance` , `summary` , `estimates` , `correlations` , `fittedvalues` , `accumulated` ); default `mode` , `summ` , `esti` , `accu` |

`FACTORIAL` = scalar |
Limit for expansion of terms; default 3 |

`DENOMINATOR` = string token |
Whether to base ratios in accumulated summary on rms from model with smallest residual ss or smallest residual ms (`ss` , `ms` ); default `ss` |

`SELECTION` = string tokens |
One or two criteria to be printed with the models (`%variance` , `%ss` , `adjustedr2` , `r2` , `dispersion` , `aic` , `sic` , `bic` ); default `%var` , `aic` , `sic` |

### Parameter

`TERMS` = formula |
Fixed terms to be dropped |

`VRDROP`

is one of several procedures designed to improve the process of determining the appropriate fixed terms to include in a `REML`

analysis. (The others are `VRFIT`

, `VRADD`

, `VRDISPLAY`

, `VRKEEP`

, `VRSETUP`

, `VRSWITCH`

and `VRTRY`

.) They do this by a generalized regression analysis, with a weight matrix based on variances estimated from the original `REML`

analysis (with the full fixed model). See `VRFIT`

for details.

The terms to be dropped are specified by the `TERMS`

parameter, in a similar way to the `FIT`

directive. The `FACTORIAL`

option sets a limit (by default 3) on the number of factors and variates in each term. Terms containing more than that number are omitted.

The `PRINT`

option controls printed output as in the regression directives, except that some irrelevant settings are omitted. (For example, `grid`

is relevant only to the fitting of nonlinear models.) See `VRDISPLAY`

for more details.

The `DENOMINATOR`

option specifies how the residual is selected for the accumulated analysis of variance By default it is taken from the model with the smallest number of residual degrees of freedom. However, you can set `DENOMINATOR=ms`

to take it from the model with the smallest residual mean square.

The `SELECTION`

option specifies the statistics to be displayed in the summary of analysis as in the regression directives, except that again some irrelevant settings are omitted. See `VRDISPLAY`

for more details.

Options: `PRINT`

, `FACTORIAL`

, `DENOMINATOR`

, `SELECTION`

.

Parameter: `TERMS`

.

### Action with `RESTRICT`

Any restriction applied to vectors used in the `REML`

analysis will apply also to the results from `VRADD`

.

### See also

Directives: `FIT`

, `REML`

.

Procedures: `VRFIT`

, `VRADD`

, `VRDISPLAY`

, `VRKEEP`

, `VRSETUP`

, `VRSWITCH`

, `VRTRY`

.

Commands for: REML analysis of linear mixed models.

### Example

CAPTION 'VRDROP example',\ 'Example 5.3.6 from The Guide to Genstat, Part 2 Statistics';\ STYLE=meta,plain FACTOR [NVALUES=322; LEVELS=27] Dam & [NVALUES=322; LEVELS=18] Pup FACTOR [NVALUES=322; LEVELS=2; LABELS=!T('M','F')] Sex FACTOR [NVALUES=322; LEVELS=3; LABELS=!T('C','Low','High')] Dose VARIATE [NVALUES=322] Littersize,Weight OPEN '%GENDIR%/Examples/GuidePart2/Rats.dat'; CHANNEL=chan READ [CHANNEL=chan] Dose,Sex,Littersize,Dam,Pup,Weight; \ FREPRESENTATION=2(labels),4(levels) CLOSE chan VCOMPONENTS [FIXED=Littersize+Dose*Sex] RANDOM=Dam/Pup REML [PRINT=model,components,wald] Weight VRFIT [PRINT=estimates] Littersize+Sex*Dose VRDROP [PRINT=estimates,accumulated] Sex.Dose